Integrated circuits (ICs) are typically formed using a silicon substrate. The ICs often include a series of active devices that are electrically connected to one another and are manufactured in or on the substrate. Each active device is typically formed by changing the conductivity of a particular region of the substrate, i.e., by implanting or diffusing impurities into the substrate.
One of these active devices, the complementary high voltage bipolar transistor (BT), has been adapted to be used in ICs requiring high performance and speedy amplification. If a complementary high voltage BT is formed on an SOI substrate to improve the electric characteristics and prevent latch-ups due to a parasitic transistor, a number of can problems occur. For example, a SOI substrate often contains an epitaxial layer on which the complementary high voltage BT is formed, which can contribute to defects of the devices. In particular, one of the defects is that the breakdown voltage (which is greatly influenced by the thickness of the epitaxial layer) can not be easily controlled.
Other problems exist for complementary high voltage BTs formed on SOI substrates. In particular, a pnp bipolar transistor (which determines electric characteristics of the device depending on relatively low hole movements) can increase the breakdown voltage in a collector region due to an over diffusion in the P+ type buried layer used as the current path for a collector in the BT.